Field of the Invention
The present invention relates to a honeycomb filter that removes particulate matter (hereinafter, referred to as “PM”) in an exhaust gas emitted from an internal combustion engine such as a diesel engine.
Description of the Related Art
An exhaust gas emitted from an internal combustion engine such as a diesel engine includes a large amount of PM such as soot mainly containing carbon polluting an environment. For that reason, a filter for removing (trapping) PM is generally mounted on an exhaust system of a diesel engine or the like.
As a filter used for such a purpose, a honeycomb filter formed of a ceramic material is widely used. In general, the honeycomb filter includes a honeycomb substrate and a plugging portion. The honeycomb substrate includes a porous partition wall defining a plurality of cells extending from an inflow end face as an end face to which a fluid flows to an outflow end face as an end face from which a fluid flows. When a plugging portion plugging one end of each cell is disposed in any end face of the honeycomb substrate, the honeycomb filter can be obtained.
When such a honeycomb filter is used to remove PM contained in an exhaust gas, the exhaust gas flows from the inflow end face of the honeycomb filter into the cell of which an end near the outflow end face is plugged. Subsequently, the exhaust gas passes through the porous partition wall and flows inside the cell of which the end is plugged at the inflow end face. Then, when the exhaust gas passes through the porous partition wall, the partition wall functions as a filtration layer and PM in the exhaust gas is trapped by the partition wall so that the PM is accumulated on the partition wall. In this way, the exhaust gas from which the PM is removed flows outward from the outflow end face.
In recent years, as one of the honeycomb filters, there is proposed a honeycomb filter in which the cross-sectional area of an exhaust gas inflow side cell is larger than the cross-sectional area of an exhaust gas outflow side cell (for example, see Patent Documents 1 and 2). The honeycomb filter with such a structure has an advantage capable of suppressing a problem in which the exhaust gas inflow side cell (the cell of which an end at the outflow end face is plugged) is blocked by PM accumulated on the partition wall.
Further, in order to suppress a breakage such as a crack caused by thermal stress in a portion near the outflow end face of the honeycomb filter when the honeycomb filter is regenerated, a honeycomb filter is proposed in which a weak portion is densified (for example, see Patent Document 3). In order to continuously use the honeycomb filter for a long period of time, the filter needs to be periodically regenerated. That is, in order to return the filter performance to the initial state by reducing the pressure loss increased by the PM accumulated inside the honeycomb filter with time, the PM accumulated inside the filter needs to be burned and removed. During the regeneration of the filter, the PM accumulated inside the filter is burned sequentially from the inflow side end face. For that reason, in the portion which is closer to the outflow side end face, the temperature caused by the heat generated at the front side (the upstream side) and the heat generated by the combustion of the PM at that portion abruptly increases. Thus, a problem arises in that the temperature in each portion of the filter increases non-uniformly and a breakage such as a crack easily occurs in a portion near the outflow end face of the honeycomb filter due to the thermal stress. In the honeycomb filter proposed in Patent Document 3, the heat capacity and the heat transfer efficiency in a portion near the outflow end face increase by densifying the portion. As a result, an increase in temperature in a portion near the outflow end face during the regeneration of the filter is suppressed and hence a breakage caused by the thermal stress hardly occurs in that portion.
[Patent Document 1] JP-B-4471622
[Patent Document 2] WO 2008/117559
[Patent Document 3] WO 2008/078799